-Note this will not improve all speakers (mine were very cheap and as pointed out by Arx the enclosure was not tuned to the maximum and adding the diaphragm improved the tuning)
-I also would like you to know that the sound produced by adding this will be different. I recommend that you compare your two options to see what sound you prefer.
Step 1: Materials and Supplies
-latex (balloon, glove, ext.)
Step 2: How to Do It
Cut a flat strip of latex big enough to cover your bass reflex air port.
Tape it tightly over the hole and be sure there are no leaks
Fire it up with these tests:http://www.stealthsettings.com/woofer-bass-subwoofer-sound-test
Number 10 is best
My mod took about 5 tries for best quality, you want the smallest square possible and as little latex contacting the tape as possible, tell me if it improves your speakers.